How about putting Sherlock on the main screen as well? Think about cruising through a WiFi hot-spot, and using Sherlock to catch the movie times, or check if the flight of the person you have to pick up at the airport is on time, then something like a link on the address to go to your GPS and give you diresctions there. I don't know how difficult the second part would be, because I'm new to the Mac game, and haven't started to look at coding, but the first part should be pretty simple, right?

I know you said the buttons aren't all done, but (and I'm not familiar with MMC, so this might be a stupid question): are you planning to add WiFi, bluetooth, GPS, Sherlock, ipod connectivity, am/fm, etc? Basically configuring it to, with added accessories, do everything you need in the car electronicly? Thanks.
p.s. love the think twice design.